Diffusion coating

ABSTRACT

Rich chromium diffusion coating on dispersion-strengthened nickel or nichrome is obtained in one step by embedding work in chromium diffusion coating pack containing nickel with or without some cobalt, and held in unsealed retort cup at least 15 inches high. Pack can also contain metallic iron to reduce coating temperature. Masking is arranged by covering the sites to be masked with a layer of a mixture of nickel powder and inert filler. For better high temperature oxidation resistance the chromium-rich coating is covered by an aluminum diffusion coating from a simple aluminum diffusion pack or one that has the aluminum mixed with chromium. Low temperature aluminum diffusion is more uniform when pack energizer is aluminum chloride or other material that does not generate nitrogen, and gives good protection against marine corrosion of steels, and particularly when there is a chromate-type coating applied over the aluminizing. Highly effective chromate-type coating mixture consists essentially of aqueous solution of chromic and phosphoric acid also containing magnesium salts of said acids and dispersed polytetrafluoroethylene particles. Aluminized superalloy can be heated in air to whiten it, then cleaned to give product having more ductile case. Such coated superalloy can also be stripped of coating by aqueous HNO3-HF-CrO3 bath.

United States Patent 1191 Baldi 1*Jan. 15, 1974 DIFFUSION COATING [75]Inventor: Alfonso L. Baldi, Drexel Hill, Pa.

[731' Assignee: Alloy Surfaces Co., Inc., Wilmington, Del.

[ Notice: The portion of the term of this patent subsequent to Oct. 9,1990,

has been disclaimed.

[22] Filed: May 18, 1972 21 Appl. No.: 254,403

Related US. Application Data [63] Continuation-in-part of Ser. Nos.219,514, Jan. 20, I972, and Ser. No. 90,682, Nov. 18, I970, each is acontinuation-in-part of Ser. No. 837,811, June 30, 1969, abandoned.

[52] US. Cl. 117/1072 P, l48/6.l6, 29/194,

Berkley l l7/l07.2 P X Primary Examiner-Ralph S. KendallAttorneyConnolly and Hutz [57] ABSTRACT Rich chromium diffusion coatingon dispersionstrengthened nickel or nichrome is obtained in one step byembedding work in chromium diffusion coating pack containing nickel withor without some cobalt, and held in unsealed retort cup at least 15inches high. Pack can also contain metallic iron to reduce coatingtemperature. Masking is arranged by covering the sites to be masked witha layer of a mixture of nickel powder and inert filler. For better hightemperature oxidation resistance the chromium-rich coating is covered byan aluminum diffusion coating from a simple aluminum diffusion pack orone that has the aluminum mixed with chromium. Low temperature aluminumdiffusion is more uniform when pack energizer is aluminum chloride orother material that does not generate nitrogen, and gives goodprotection against marine corrosion of steels, and particularly whenthere is a chromate-type coating applied over the aluminizing. Highlyeffective chromate-type coating mixture consists essentially of aqueoussolution of chromic and phosphoric acid also containing magnesium saltsof said acids and dispersed polytetrafluoroethylene particles.Aluminized superalloy can be heated in air to whitenit, then cleaned togive product having more ductile case. Such coated superalloy can alsobe stripped of coating by aqueous l-INO -HF-CrO bath.

13 Claims, 2 Drawing Figures PAIENTEUJAN 1 51914 DIFFUSION COATING Thepresent application is in part a continuation of applications Ser. No.219,514, filed Jan. 20, 1972, and Ser. No. 90,682, filed Nov. 18, 1970,each of which is in part a continuation of application Ser. No. 837,811, filed June 30, 1969 and subsequently abandoned.

The present invention relates to the coating of TD nickel and other hightemperature materials to make them more resistant to exposure.

Thoria-dispersed nickel, more commonly called TD nickel, andthoria-dispersed nichrome, similarly called TD nichrome, are unusualmaterials, particularly at high termperatures. They have greaterstructural strength at temperatures of about 2,300F than nickelandcobalt-based superalloys. However it has been difficult toadequatelyprotect the TD nickel and TD nichrome against excessiveoxidation at such high temperatures.

It has been known that good oxidation protection is provided by firstdiffusion coating the TD nickel with 40 to 60 milligrams chromium persquare centimeter of surface, followed by a diffusion coating of 1.5 to4 milligrams aluminum per square centimeter. Unfortunately uniform heavysmooth chromium coatings of desirable structure have been difficult toobtain .in a readily reproducible satisfactory form without using twoseparate chromium diffusion operations.

Among the objects of the present invention is the provision of improvedcoating techniques for TD nickel and related materials.

The foregoing as well as other objects of the present invention will bemore fully understood from the following description of several of itsexemplifications, reference being made to the accompanying drawings inwhich:

FIG. 1 is a vertical sectional view of a diffusion coating set-uppursuant to the present invention; and

FIG. 2 is a similar view of a related masked diffusion coatingoperation.

According to the present invention a rich chromium layer of high qualityis diffused into TD nickel or TD nichrome articles in one step from apack held in an unsealed retort cup at lcast l5 inches high, and thepack contains 5 to 40 percent chromium by weight as well as nickel in aproportion from about one-third toabout one-twentieth of the chromium byweight. The pack can also contain cobalt in an amount from aboutonetwentieth to about one-fifth the chromium by weight. Any places wherethe articles are not to be coated can be masked as by an interveninglayer of nickel powder diluted with alumina.

A layer of aluminum can then be diffused in over the chromium-diffusedcase. This aluminum diffusion can be a simple pack aluminizing or can bethe more complex aluminizing from packs that contain chromium as well asaluminum, such as shown in Canadian Pat. No. 806,618 or U. S. Pat. No.3,257,230. Some of the chromium from such complex packs diffuses alongwith the aluminum into the surface being coated, but the extra chromiumis not significant where that surface already contains a large contentof chromium. For low temperature aluminizing, that is where aluminumdiffusion is carried out at 1,300F or below, more uniform coatings areobtained when nitrogen is excluded, as by using an activator such asaluminum chloride that does not liberate nitrogen. Manganese can also beadded to analuminum diffusion coating pack for coating ferrous metalssuch as stainless steels, and it helps provide a coating that isprotective against marine forms of corrosion, expecially when suchcoating is covered by a chromatetype of coating. Even without themanganese addition very good protection is obtained.

- Turning now to the drawings, the apparatus of P16. 1 has been foundvery effective for diffusion coating in one step more than 40 milligramschromium in each square centimeter of the surface of TD nickel jetengine burner rings, and to produce a high quality dense coated surfacewith very few or no inclusions. This apparatus has a bell-type furnaceshell 10 placed around an outer retort 12 also of bell type and with itsopen bottom set on a sealing strip 14 over a retort base 16 embedded ina concrete floor 18. The outer rim 20 of the base is a water-coolingjacket to protect the sealing strip 14, and the center 22 of the baseprovides a bottom closure for the outer retort 12. Base center 22 hasconduits 31, 32, 33 sealed through it for passing gas flushing lines 41,42 as well as thermocouple leads 43, and can be made hollow so that itcan be pressurized with gas and connected to a pressure gauge thatindicates when any leak develops.

Above the retort base 16 there is placed a spacer 50 which can merely bein the form of a spider-like set of welded-together metal wallsradiating from a center and spaced from each other to permit passage ofthe gas flushing and thermocouple lines. Above the spacer is mounted aplurality of inner retorts 51, 52 and 53 each in the form of an annuluswith an open top 59. These inner retorts are shown as having cylindricalouter and inner walls 61 62 and an annular bottom sheet 63, the partsbeing dimensioned so that the jet engine deflector rings will bereceived in them. Such rings can have a wall thickness of about 40 milswith diameters of from about 10 to about 30 inches, and for such sizesthe diameter of inner cylindrical wall 62 should be significantly lessthan 10 inches while the diameter of the outer cylindrical wall 61should be significantly greater than 30 inches. The upper edges of theseinner retorts have grooves or notches 67 that can act as vents, and theinside height of an inner retort between its floor 63 and the bottom ofnotches 67 is at least 15 inches, preferably at least 18 inches. lt isalso helpful to have a loosely fitted cover 71 over each inner retort.

A chromium diffusion operation is carried out by loading the innerretorts with the burner rings packed in a chromium diffusion pack suchas one containing by weight 20 percent chromium powder, 3 percent nickelpowder, 3 percent cobalt powder, 0.5 percent ammonium chloride powder,the balance being alumina powder. The packed retorts are then stacked onthe spacer 50, the outer retort l2 lowered over them, a stream of argonstarted through the flushing line 41, and the furnace shell 10 thenplaced around the assembly.

When the air in the retorts has been displaced by the argon the furnaceis started and the flushing line 41 switched from an argon supply to asupply of dry hydrogen. The flushed gases exiting through line 42 canthen be lit where they discharge from the exit conduit. The

-rsts ls a brsss t "P. r ysth tvtnsss t N to 2,200F and kept there for20 to 30 hours under the control of one or more thermocouples. The heatis then shut off and the assembly permitted to cool. When thetemperature in the inner retorts drops below 300F the hydrogen flush isreplaced by an argon flush. After the hydrogen is flushed out theretorts can be opened and unloaded. The coated burner rings can then begiven a light dry blast with 100 grit aluminum oxide at 20 to 30 poundsper square inch, to clean up their surfaces, leaving them very smoothwith a surface chromium content about 35 to 40 percent by weight and achromium pick-up of about 50 milligrams per square centimeter ofsurface. Little or no alpha chromium phase and inclusions are present inthe coating, and it is highly suited for receiving an aluminum diffusioncoatmg.

The foregoing results are obtained with inner retorts 15 inches high.However when the heights of these retorts are inches and the processotherwise identical, the pick-up of chromium is only about half theforegoing amount. With retort cups deeper than inches the chromiumpick-up is slightly higher than with the 15 inch depths. The coatingtemperature should be at least 2,l50F to keep the coating time frombeing too long. After about 22 hours at 2,l50 to 2,200F the coating ratediminishes sharply and further treatment tends to post-diffuse thedeposited chromium rather than deposit significantly more chromium.

With the cobalt-nickel-chromium pack mixture the coating temperatureneed not be any higher than 2,150F to obtain best results. When nickelis used in the pack without cobalt, it is preferred to have the coatingcarried out at 2,200F. Unless nickelis present in the pack in an amountat least about one-twentieth the weight of the chromium, thediffusion-coated case is apt to contain undesirable inclusions.

The presence in the coating pack of about I to 10 percent metallic ironby weight speeds up the chromium deposition, and permits the use ofcoating temperatures 25 to 100F lower than indicated in the aboveexample. However the coating then tends to be somewhat rougher and alsocontains some inclusions. The iron can be added in the form of puremetal such as carbonyl deposited iron powder, as ferrochrome or Ni-Fe orCr-Ni-Fe alloy.

Varying the chromium content of the pack from about 5 to about 40percent changes the surface chromium content of the coatings but doesnot detract appreciably from the quality of the coatings. Varying thenickel content between about one-twentieth to about one-third of thechromium by weight generally shows that the coating has fewestinclusions when the nickel content is between about one-tenth to aboutone-fourth of the chromium by weight. However better oxidationresistance and a faster coating rate is provided by the added presenceof cobalt in an amount from about onetwentieth to about one-fifth theweight of the chromium. Because of such faster coating the coatingtemperature can be 50F lower with the cobalt-containing packs than withcorresponding cobalt-free packs. The cobalt diffuses into the coatingwith the chromium, when both are present in the pack, and its presencein the coating is evidenced by the typical beige color of cobaltaluminide which appears after the subsequent aluminizing. The alumina inthe pack can be replaced by any other inert filler such as kaolin,magnesium oxide or C 0 and such fillers can be used singly or in anycombination. The coating step can also be performed in other types ofequipment, so long as the minimum retort height is observed.

TD nichrome containing about 20 percent chromium, 2% Th0 the balancenickel, can be similarly diffusion coated with chromium. Good resultsare obtained with the TD nichrome when the chromium pickup in thecoating is only about 30 to about 40 milligrams per square centimeter.However, the diffusion of chromium into the TD nichrome is slower thanin TD nickel so that about the same treating temperatures and times areused. Other forms of dispersion-strengthened nickel and nichromes suchas zirconia-strengthened and hafnia-strengthened nickel show the sametype of improvement when coated in the above manners.

activator that greatly speeds up the coating operation.

Any other activator such as ammonium iodide, ammonium bromide, ammoniumfluoride, ammonium bifluoride, elemental iodine, elemental bromine,hydrogen bromide, or the higher aluminum halides (chloride, bromide andiodide) can be used individually or in any combination, and inconcentrations from about 0.05 percent to about 1 percent by weight.With a pack that is not perfectly anhydrous, the minimum activatorcontent should be 0.1 percent.

The pack, or at least those pack ingredients that remain solid duringthe coating, is of relatively fine particle size. The maximum particlesize is desirably microns and preferably less than 40 microns, althoughthe activator and scavenger particles can be up to about 1 millimeter insize without detracting from the quality of the coating. Best resultshave been obtained with metal particles less than 10 microns in size andwith filler particles up to about 40 microns in size.

The chromium diffuion packs also give better results for the second andsubsequent coating treatments after they are freshly mixed. lf desiredthe freshly mixed packs can be subjected to a blank run without workpieces before they are placed in service. Used packs are simply reusedwith the addition of another charge of activator so long as the metalcontent is adequate. About 1% to 2 percent chromium can also be added toused packs to keep the metal content substantially unchanged throughsuccessive coating runs. Where cobalt is a pack ingredient substantialamounts are consumed by the coating and should be replaced as by addingone-eighth to one-half percent after each coating run. Any nickeladditions should be kept very low inasmuch as there is very littlenickel consumed during the coati W M. W "a, a

It is not necessary to use a hydrogen flush or hydrogen atmosphereduring the diffusion. An argon flush maintained throughout the coatingtreatment also gives good results although it is more awkward to monitorthe argon flow because it does not burn. Indeed no flush whatever isneeded in which case the outer retort can be sealed against its cover asby a molten glass seal.

The YSFeEbihEEhmmium coating operation adds enough chromium to the workpiece surface to significantly increase its height. At locations such asattach- .ment sites and the like where the greatest dimensional accuracyis needed, the work pieces can be masked as by a layer or pocket of amixture of equal parts by weight inert filler and nickel powder.

FIG. 2 illustrates one masking arrangement in the coating of jet burnernozzles 100 having threaded shanks 102 which are to remain uncoated. Inan inner retort cup 104 there is placed a layer 106 of masking compounddeep enough to receive the entire shank 102 of a nozzle. This layer canbe a mixture of to 60 percent nickel powder and 40 to 90 percent aluminaor other inert filler, by weight. Over layer 106 another layer 108 ofmetal-free inert filler is placed, this being a thin layer to keepparticles from sintering in a groove 103 in the nozzle sidewall. Anotherthin layer 110 having a nickel-containing composition like that of layer106 tops off the masking combination.

A group of nozzles 100 is first pressed with th eir threaded shanks downinto the layer 106, layer 108 is sprinkled and rolled over the surfaceof layer 106 and then covered with layer 110 until the shanks arecompletely submerged, leaving the upper portions of the nozzlesprotruding up from layer 110. There is then poured-in a layer 112 ofdiffusion coating material that completely covers the nozzle tops.

Where the nozzles are TD nickel or TD nichrome and a heavy diffusioncoating of chromium is desired, the retort cup 104 should be at leastl5inches high or should be set into retort cups at least inches high asexplained above. With other materials or for lighter coatings the retortcup need only be shallow enough to hold a single layer of nozzles. Layer108 of FIG. 2 is free of metal and used where the work pieces haverecesses that might be difficult to clean up after the coating.

Where the nozzles are not subjected to an extremely Even where the workpieces to be masked are nickelbase or cobalt-base superalloys, it issometimes desirable to use a thin layer such as layer 110 of maskingmixture containing only nickel and filler at the edge of the masked areaadjacent the area to be coated. The diffusion coatings ordinarilyapplied have a substantial ability to throw a short distance and it onlytakes about a H152 to about a 1/16 inch layer of such nickelfillermixture to provide a sharp coating boundary and keep the diffusioncoating from throwing where it is not wanted. The nickel-tiller layermight have a tendency to weaken the superalloy but the extreme thinnessof such a barrier layer minimizes the weakening effect.

After the heavy chromium coating above has been completed and the workpiece surface thus coated cleaned up, the material is ready for thediffusion coating of aluminum. This is readily accomplished with analuminum-containing pack such as a mixture of 50 to 98 percent aluminaand 2 to 50 percent aluminum by weight, using a coating temperature of800 to l,l00F and a coating time of about 4 to about 24 hours. Suchaluminizing is an alternative to the more complex aluminizing referredto above as described in Canadian Pat. No. 806,618 and U. S. Pat. No.

3,257,230. The more complex type of treatment is preferably conducted atl,450 to 1,550F for about 6 hours when it follows the above chromizing.

A good form of simple aluminizing is accomplished with a pack consistingof percent alumina and 30 percent aluminum, both 325 mesh, activatedwith A percent aluminum chloride, using a coating temperature of 850 Ffor 20 hours. Another good example of a pack contains percent of.thealumina and 20 percent of the aluminum powder, with'the same activatorin the same concentration, used at 800 F. It is particularly desirableto keep the temperature below 900 F during this coating treatment. Thereis no minimum retort height preference for aluminizing, but thealuminizing can be carried out in the apparatus of FIG. 1, or in anyother form of diffusion-coating apparatus. The coating produced by asimple aluminizing pack gives better results when the pack has beenpreviously used in a coating run. It is accordingly helpful whenstarting with a fresh pack to give it a break-in treatment with a dummywork piece, or even with no work piece at all.

The simple aluminizing described above does not produce a consistentlyuniform coating when an ammonium halide is used as the energizer and thematerial being coated is an age-hardenable or a martensitic stainlesssteel. The lack of uniformity appears to be due to the presence ofnitrogen in the retort atmosphere during the coating, and the resultanterratic formation of nitrides. The aluminum chloride energizer does agood job of flushing out residual air without introducing nitrogen, butother energizers such as elemental iodine and bromine, iodinetrichloride or similar nitrogen-free halogen compounds including otherhigher halides of aluminum (chloride, bromide or iodide), halides ofsilicon, colombium, titanium, boron, zirconium, hafnium, tantalum,chromium, molybdenum, tungsten, iridium, osmium, platinum, gallium,germanium, tin and phosphorus will do the same although they are notpreferred. Whichever energizer is used is preferably in an amount fromabout 0.1 to about 1 percent of the pack weight. Also better results areobtained if the unvaporized energizer is isolated from the work piecesas by enclosing all the energizer in a container that permits the escapeof vapor. A container for this purpose can be made of fine screening orwith an open top or with a loosely fitted top and several of suchcontainers can be distributed throughout the mix. Such a container orcontainers can be embedded in the coating pack and will release vaporsof energizer as the pack is heated up to coating temperature, suchvapors accomplishing the same flushing and depositaccelerating resultsexpected of an energizer, but without the coating flaws experienced whensolid aluminum chloride is mixed into the entire pack. The containerholding the energizer can be made of plain carbon steel or othersuitable metal such as aluminized steel or low alloy chromium steel, oreven martensitic stainless steel. The retort itself can also be made ofany of the foregoing materials.

As an alternative packing technique all the energizer can be confined toa stratum of the pack below the work pieces, with the remainder of thepack being a uniform mixture of filler and diffusing material. Thus goodresults are obtained when the diffusion retort is first packed withabout a k to 1 inch deep layer of the pack material, all the energizeris then sprinkled over that layer, another 1 inch deep layer ofenergizer-free pack placed over the foregoing, and the retort thenfilled with work pieces and additional pack. However it is simpler topack the retort with the separately contained energizer, and work piecescannot be inadvertently inserted in such a separately containedenergizer. Should a work piece be accidentally pushed into theseparately stratified energizer of the alternative packing technique, agood coating will not form on the portion of the work piece that haspenetrated into that stratum.

In general the simple aluminizing as well as the more complexaluminizing are effectively used to cause an aluminum pick-up of about0.5 to 7.5 milligrams per square centimeter of surface coated, giving acoating case about 0.1 to about 1.5 mils thick. A preferred pick-uprange is from about 1 to about 5 milligrams per square centimeter. Thecoating packs used can be replenished as by adding 1 percent aluminumafter every use, even after a break-in use.

A useful diffusion aluminizing of stainless steel and chromium steels isalso effected by incorporating with the aluminum about one-fourth tothree-fourths metallic manganese calculated on the weight of thealuminum. Thus a diffusion coating pack of 60 percent alumina, 30percent aluminum and 10 percent manganese will give at 875 F over aperiod of 10 hours an alumi' nized coating on age-hardenable ormartensitic stainless steels that provide good protection, particularlyagainst marine-type corrosion. Type 410 stainless steel jet enginecompressor blades or gas generator housings given a 1 mil thick coatingcase from a manganese-free aluminum pack at temperatures from 800 to 1,lF will however withstand corrosion in salt air for a particularly longperiod of time.

A chromate-type coating applied over the manganese-containing aluminumdiffusion coating or the manganese-free aluminum coating, furtherincreases corrosion resistance. A particularly effective chromatetypecoating for this purpose is one that is made by dipping the aluminizedcompressor blade after vapor honing to clean the surface, into anaqueous solution of phosphoric acid and chromic acid containing perliter about to 100 grams phosphoric acid and about 1 to 25 grams chromicacid, removing the dipped blade and permitting the solution to drain,followed by calcining the blade with the residual coating solutionthereon at 800 F for minutes. So-called conversion coatings such asdescribed in U. S. Pat. No. 3,385,738 are not sufficiently protective atelevated temperatures, that is at about 800 F or higher. The chromicacidphosphoric acid coatings of U. S. Pat. application Ser. No. 90,682filed Nov. 18, 1970, with or without the related treatments theredisclosed are much better in this respect and provide protection attemperatures that reach as high as 1,200F.

Very effective results on aluminized greek ascoloy are obtained with l0to grams CrO and 57 grams orthophosphoric acid per liter, the calciningbeing at 600 F for 40 minutes. In general calcining temperatures canvary from about 450 to about 900 F, and should be long enough to causethe chromate-type coating to become almost completely (at least about 90percent insoluble in water.

Even better results are obtained with age-hardenable and martensiticstainless steels when the chromate-type coating also contains magnesiumas well as particles of polytetrafluoroethylene, as in the followingexample:

EXAMPLE 180 grams CrO 130 grams MgO 410 cc 85% l-l PO (by weight inwater) 15 cc aqueous dispersion of polytetrafluoroethylene particlesless than 1 micron in size, containing 13.5 grams of the resin, and

Water to make up 3 liters of coating bath.

The MgO dissolves in the acid and remains dissolved upon dilution, whilethe resin particles remain undissolved but dispersed. If the stock resindispersion is sensitive to acid, it is added after the MgO is dissolvedinasmuch as this sharply lowers the acidity. The resin particles neednot be very stably dispersed, although such stability is improvedthrough the use of a small amount of a dispersing agent that is notsensitive to acid or oxidizers. Non-ionic surface-active agents orquaternized imidazoline surface-active agents such as N CH; R-PJIk-CIhCOOH are suitable for this purpose. In any event the bath can beagitated to assure uniformity of dispersion.

Dipping an aluminized or uncoated greek ascoloy compressor blade in thebath of this example at room temperature, followed by oven heating at700 F for minutes provides a cured coating weighing about 0.27milligrams per square centimeter that gives excellent protection inmarine environments.

The ingredients of the bath of the foregoing example can vary asfollows:

Magnesium 0.4 to 1.7, preferably 0.9 to 1.4 mols per liter Chromate ion0.2 to l, preferably 0.4 to 0.8 mols per liter Phosphate ion 0.7 to 4,preferably 1.5 to 3.5 mols per liter Resin 2 to 14, preferably 4 to 10 gper liter Coating weights above about 0.5 milligram per squarecentimeter tend to craze, and below about 0.2 milligram per squarecentimeter are not as effective although as little as 0.05 milligram ofcoating per square centimeter givesnoticeably improved corrosionresistance. This improvement increases with increased coating weight,and two coats can be used if desired, as by going through a second suchcoating treatment after a first coating is applied and cured, to make atotal chromate-type coating weight of about 1 milligram or more persquare centimeter.

Phosphorous acid and other phosphorus acids like pyrophosphoric acid canbe substituted for part or all of the orthophosphoric acid withoutsignificantly lowering the effectiveness of the chromate-type coating.

The foregoing chromate-type coatings greatly prolong the useful livesofjet engine compressor blades of martensitic stainless steels that havea simple aluminized case, particularly in the salty air of marine usewhere higher temperatures are encountered. Other ferrous metals such asaustenitic stainless steels and even low alloy and plain carbon steelsthat have aluminum diffusion cases are also made much more resistant tomarine corrosion by the foregoing chromate-type coating. However withlow alloy and plain carbon steels the resistance to marine corrosion isfurther increased if before the aluminizing the metal is given achromium or mixed chromium-nickel diffusion case or even a chromiumplating. ln general it is preferred for the ferrous-surface to containat least percent chromium before aluminizing, but sharply improvedresults are obtained with AlSl 4140 steel or when as little as l percentchromium is present in the surface of a steel.

Applyingdiffusion coatings on some high strength metals like type 410stainless steel can cause loss of strength, particularly if the coatingis applied at l,000

. F or higher, and the coated material is slowly cooled fected by theprocess disclosed in U. S. Pat. application Ser. No. 159,175 filed July2, 1971.

First stage turbine vanes of cobalt-base or nickelbase superalloys thathave been aluminized with either the complex or simple aluminizing, arealso improved by heating in air at 2,050" to'2,l00 F until their surfaceis uniformly whitened, generally about 10 to 20 hours, then glassblasting after cooling to remove the white skin (which appears to bealuminum oxide). The resulting vane looks very much like the untreatedaluminized vane',.except for a loss of some color'where the vane is acobalt-base superalloy like WI 52, but the aluminized case is moreadherent and less subject to spalling and the like on handling. Wherethe case is damaged it can be stripped off by the processes described inU. S. Pat. Nos. 3,622,391 granted Nov. 23, 1971 and 3,458,353 grantedJuly 29, 1969. Those processes are further improved by modifying theaqueous HF-HNO; stripping baths thus used so that CrO is also present inthose baths. A solution of g HF, 80 g HNO and 5 g CrO in 920 g watermakes a very effective stripping bath for this purpose when used at 85F. The aluminized case dissolves in the bath and the base metal is notsignificantly attacked. Polished surfaces of the substrate survive thestripping bath treatment without much loss of polish.

The Cro -containing stripping baths can also contain other ingredientsthat do not detract from its effectiveness. Ammonium, alkali metal andalkaline earth metal cations as well as acetate and phosphate anions areexamples of such allowable addition to the baths. In general the HFcontent can vary from about 0.1 to 5 percent by weight, the HNO content3 to percent by weight, and the ratio of HF to CrO from 15:] to 1:5 byweight. Preferred ranges are HF 0.5 to 3% HNO 5 to 15% HFzCrO 7:2 to 1:2all calculated by weight. All of the stripping baths work well at from50 to 140 F.

Any attack on the superalloy base caused by the CrO -containing or CrO-free baths is further minimized by keeping the work piece beingstripped in contact with metallic nickel or cobalt. Thus the strippingcan be carried out by placing the work pieces to be stripped and thestripping bath in a nickel-surfaced container, or holding the workpieces in a nickel wire mesh basket while they are dipped in thestripping bath.

Pickling inhibitors can also be added to the stripping bath.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is, therefore, to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed:

1. In the process of diffusion coating articles ofdispersion-strengthened nickel or dispersion-strengthened nickel alloyscontaining about 20% chromium, with a deep layer rich in chromium from adiffusion coating powder pack in which the coating is effected with thepack held in an unsealed retort cup at least l5 inches high, and thepack contains about 5 to about 50% chromium as well as nickel in aproportion from about onethird to about one-twentieth of the chromium.

2. The combination of claim 1 in which all the pack ingredients thatremain solid during the coating have particle sizes smaller thanmicrons.

3. The combination of claim 1 in which the metalingredients of the packhave particle sizes smaller than 40 microns.

' 4C Th e contain-anon of clainii'inwhi h the backin contains cobalt inan amount from about one-twentieth to abOutpne fifththe weight of thechromium.

5. In the process of protecting articles of dispersionstrengthenednickel by diffusion coating them with chromium and then with aluminum,the improvement according to which the chromium coating is applied as asingle coating step inaccordance with claim 1.

The combination of claim 1 in which the pack also contains cobalt in anamount from about one-twentieth to about one-f fth the weight of thechromium.

9. Dispersion-strengthened nickel which has been coated bythe process ofclaim f8,

l0. ln the process of protecting articles of dispersionstrengthenednickel or dispersion-strengthened nickel alloys containing about 20percent chromium by diffusion coating them with chromium and then withaluminum, the improvement according to which the diffusion coating withchromium is effected with a pack containing about 5 to about 50 percentchromium, nickel in a proportion about one-third to about onetwentiethof the chromium, and cobalt in a proportion about one-fifth to aboutone-twentieth of the chromium.

11. The product produced by the process of claim l0.

12. In the process of protecting dispersionstrengthened nickel articlesby diffusion coating them with chromium, the improvement according towhich the diffusion coating with chromium is effected with a packcontaining about -5 to about 50 percent chromium, nickel in a proportionabout one-third to about one-twentieth of the chromium, and cobalt in aproportion about one-fifth to about one-twentieth of the chromium.

/ l3. The product produced by the process of claim 12. |K 1

2. The combination of claim 1 in which all the pack ingredients that remain solid during the coating have particle sizes smaller than 150 microns.
 3. The combination of claim 1 in which the metal ingredients of the pack have particle sizes smaller than 40 microns.
 4. The combination of claim 3 in which the pack also contains cobalt in an amount from about one-twentieth to about one-fifth the weight of the chromium.
 5. In the process of protecting articles of dispersion-strengthened nickel by diffusion coating them with chromium and then with aluminum, the improvement according to which the chromium coating is applied as a single coating step in accordance with claim
 1. 6. In the process of diffusion coating articles of dispersion-strengthened nickel or dispersion-strengthened nickel alloys containing about 20 percent chromium, with a deep layer rich in chromium from a diffusion coating pack in which the articles are embedded, the improvement according to which the pack contains about 1 to about 10 percent iron and the coating Temperature is below 2,100*F.
 7. The combination of claim 1 in which the diffusion coating pack also contains about 1 to about 10 percent iron and the coating temperature is below 2,100*F.
 8. The combination of claim 1 in which the pack also contains cobalt in an amount from about one-twentieth to about one-fifth the weight of the chromium.
 9. Dispersion-strengthened nickel which has been coated by the process of claim
 8. 10. In the process of protecting articles of dispersion-strengthened nickel or dispersion-strengthened nickel alloys containing about 20 percent chromium by diffusion coating them with chromium and then with aluminum, the improvement according to which the diffusion coating with chromium is effected with a pack containing about 5 to about 50 percent chromium, nickel in a proportion about one-third to about one-twentieth of the chromium, and cobalt in a proportion about one-fifth to about one-twentieth of the chromium.
 11. The product produced by the process of claim
 10. 12. In the process of protecting dispersion-strengthened nickel articles by diffusion coating them with chromium, the improvement according to which the diffusion coating with chromium is effected with a pack containing about 5 to about 50 percent chromium, nickel in a proportion about one-third to about one-twentieth of the chromium, and cobalt in a proportion about one-fifth to about one-twentieth of the chromium.
 13. The product produced by the process of claim
 12. 